1. Brief Description of the Invention
The present invention relates to improved thickening compositions, often referred to as theological additives, used to provide viscosity control and other rheological properties to aqueous systems. In many commercial theological additives, an active thickening chemical such as a polyurethane polymer is mixed with water and a volatile organic liquid to provide users with a pourable, liquid, thickening composition which can be conveniently added to aqueous systems. The present invention involves the substantial elimination of the volatile organic liquid component and its replacement with a non-volatile component, resulting in a thickening composition with equal or improved viscosity properties and with no, or with substantially diminished volatile organic content, thus having a positive impact on the environment.
2. Description of the Prior Art
It has long been known in the art to use various materials as thickening compositions to thicken aqueous systems. Aqueous systems include both water-based and latex-based paints, coatings, inks, construction materials, cosmetics, and wood stains utilized in various aspects of a civilized industrial society. Depending on the composition of the aqueous system the products made with these thickeners can be useful as decorative and protective coatings, paper coatings, cosmetics and personal care items, adhesives and sealants, inks, petroleum drilling fluids, joint compounds, and the like.
Many thickeners are known, natural, modified--natural and synthetic. Natural thickeners, for instance, include casein and alginates. Modified--natural thickeners include modified celluloses, including methyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, and carboxymethyl cellulose. These products vary in their thickening efficiency and flow and leveling properties.
Synthetic thickeners in recent years have assumed increased importance, particularly in the viscosity control of aqueous and latex paints and coatings. The synthetic thickener serves several roles in aqueous systems. In latex and aqueous paints and coatings, for instance, the thickener provides improved stability and pigment suspension, and improved application properties. In cosmetics and personal care items, the thickener improves body, feel, smoothness and silkiness, making the product more aesthetically pleasing. In petroleum drilling fluids, the use of a thickener improves the suspension of the cuttings, thereby increasing the efficiency with which the cuttings can be removed from the deep bore holes encountered in drilling, thereby extending the life of the drill bit.
Synthetic rheological thickeners include various acrylic polymers and maleic anhydride copolymers. Two patents issued to RHEOX Inc., a subsidiary of NL Industries, Inc., describe a family of polyurethane thickening compositions sold under the commercial designations RHEOLATE 244, RHEOLATE 255 and RHEOLATE 278. These patents, U.S. Patent Nos. 4,499,233 and 5,023,309 describe these synthetic thickeners as the polyurethane reaction product of polyisocyanates, polyether polyols, modifying agents which contain at least two active hydrogen moieties, terminated by a chemical capping agent.
One type of synthetic thickener used extensively in commercial applications is a low molecular weight polyurethane characterized by hydrophobic groups interconnected by hydrophilic polyether groups, and is disclosed in Rohm & Haas U.S. Pat. Nos. 4,079,028 and 4,155,892. Typically these materials have molecular weights of 2500 grams/mole or higher. The polyurethane is described as prepared by reacting at least one water soluble polyether polyol with at least one monofunctional hydrophobic organic compound. There are also present in the reaction mixture insoluble organic polyisocyanates, or polyhydric alcohol, or polyhydric alcohol ethers.
One similar type of synthetic thickener is a water-soluble, thermoplastic organic polymer having a number of monovalent hydrophobic groups incorporated in the internal portion of the polymer molecule. U.S. Pat. Nos. 4,496,708 and 4,426,485, issued to Union Carbide Corporation, describe such thickeners as water-soluble comb polymers containing a number of pendant internal hydrophobic groups separated from the uncapped end of the polymer by polyether Units.
A number of patents issued to BASF Corporation describe a family of polyether liquid thickeners for aqueous liquids obtained by capping a straight-chain polyoxyalkylene compound derived from ethylene oxide and an alcohol with at least one other lower alkylene oxide, e.g., a poloxyalkylene compound initiated with an active hydrogen-containing initiator and with an alpha-olefin oxide group. See U.S. Pat. Nos. 4,288,639, 4,354,956 and 4,904,466.
Cross linked polyesters, polyester-urethanes and polyether-alpha olefins as well as a variety of other types of polymers have also been used as synthetic thickeners. See, for example, U.S. Pat. Nos. 4,310,436 and 4,709,099 issued to BASF Corporation.
The above-described rheological additives, useful in aqueous systems, have been often referred to by the term "associative" thickeners. Associative thickeners are so called because the mechanism by which they thicken may involve hydrophobic associations between the hydrophobic species in the thickener molecules and other hydrophobic surfaces, either on other thickener molecules, or on molecules in the system to be thickened. The different types of associative thickeners include, but are not limited to, polyurethanes, hydrophobically-modified alkali soluble emulsions, hydrophobically modified hydroxyethyl cellulose or other products, and hydrophobically modified polyacrylamides.
The molecular weight and HLB of these associative thickeners, which usually are water soluble or dispersible polymers, is chosen to be sufficiently high to impart desired rheological properties to an aqueous composition containing the thickener. Advantageously, the water-soluble polymer has a molecular weight sufficiently high such that a solution containing up to 2-3 weight percent of this polymer will exhibit a viscosity of at least 5,000, preferably at least 15,000, and most preferably at least 20,000 centipoises (as measured on a Brookfield viscometer with a number 3 spindle at 10 RPM at 25.degree. C.). As explained above, a variety of polymeric materials may be used as a water-soluble polymer, including cellulose ethers, polyacrylamides, sulfonated polystyrenes, copolymers of acrylic acid, hydroxypropylated guar, and the like. The choice of the particular water-soluble polymer depends primarily on the compatibility of such polymer with the other components of the thickened system that contains the thickener, and the ultimate end use of the aqueous system.
Levels of the rheological thickening additive varying between 0.1% and about 10%, based on the total weight of the system to be thickened, have been found to be useful. As a general class, associative thickeners provide a combination of properties not found in any other single class of known thickeners. For example, they are generally nonionic, and in many cases are highly efficient viscosity improvers even though they can have middle range molecular weight. They are usually stable to water and are not sensitive to biodegradation. Associative thickeners can be further classified as molecules containing hydrophobic moieties, whether localized or dispersed throughout a hydrophilic backbone. The hydrophobic parts cause the association, either with themselves or by association with hydrophobic moieties contained in the various ingredients commonly found in aqueous paints. Typical backbones would include polyurethanes, polyethers and starch-type molecules. They are versatile in that not only do they thicken virtually unlimited types of aqueous systems, but they also impart many beneficial auxiliary properties. Thus, as additives to textile binder compositions, they actually soften rather than harden the fabric. In latex paints, especially, they not only thicken but in many cases also provide superior flow and leveling, and give excellent viscosity control under both low and high shear conditions.
Water-soluble thickeners for aqueous systems are often prepared and have been used as thickeners in a dry, solid form. Problems associated with the use of solid polymers however include poor dispersibility when added to aqueous systems, and undesirably long dissolution times. In addition, the dust associated with the incorporation of dry polymers presents similar conventional handling problems as are encountered with other types of particulate materials. Water-soluble solid polymers can be hygroscopic, and can absorb water from the air, which can cause agglomeration of the particles. Such particles are then difficult to disperse in an aqueous system. When added to aqueous systems, solid, water-soluble polymers by their nature tend to agglomerate to form clumps. Agglomeration can be reduced in many cases by adding the solid polymer to the aqueous system slowly with agitation. Such slow dissolution, however, can affect the efficiency of specific manufacturing operations.
Solid water-soluble polymers are also known to be difficult to handle in industrial processes because they require long periods of time to dissolve. Both in simple aqueous solution and, more particularly, in formulations comprising other chemicals and ingredients, extended agitation and aging periods are required before proper incorporation is attained. When polymers are furnished as concentrated, nearly solid solutions, they are difficult to dilute uniformly, so as to avoid the production of local areas of high concentration. This requires users and formulators to develop crude empirical methods for estimating the final rheological properties of the formulations, and to inventory excess amounts of polymer for reprocessing if the final rheological properties of the original compositions do not meet the specific required specifications.
For the above reasons, manufacturers have searched for and insisted on a fast, effective and simple way of incorporating water-soluble polymers into aqueous systems. Because of this, most commercial associative thickeners are today sold as pourable liquids. Such commercial rheological additives in liquid form for latex paint and other compositions usually involve preparing the thickening composition as a mixture of water and a water miscible organic solvent, such as diethylene glycol monobutyl ether (also known as butyl Carbitol.TM.) or ethylene or propylene glycol, into which the associative thickener is mixed. The main reason for the addition of this organic cosolvent is to lower the viscosity of these polymers in water to provide ease in handling and allow a practical level of activity in the final product. Typically, the viscosity of the thickening composition should be less than 15,000 cP (at 10 RPM), at 25.degree. C. so that it will readily drain from the storage container, be pourable, and rapidly incorporate into the composition to be thickened at room temperature. The water miscible solvent selected for such commercial compositions has, up to this time, exclusively been an organic solvent.
The use of a volatile organic cosolvent contributes to the Volatile Organic Content, designated VOC, of the aqueous system that was thickened. The reduction of the VOC of water-borne systems, including important paint and construction systems, without adversely affecting the other properties of the systems, is currently a very active research topic throughout the world, but most particularly in the United States. Although rheological additives are used at relatively low levels in paints, they contribute to the total paint VOC, because they are typically sold in the aforesaid organic solvent/water mixtures. For example, commercial polymeric theological additives for water-borne paint systems, designated RHEOX RHEOLATE 255, 278, and 300, are currently sold as liquids in a thickener/butyl carbitol/water mixture. The butyl carbitol is an organic solvent added to among other things lower the viscosity of the water soluble polymers in water, and thus allow a practical level of activity in the final product. The aforementioned Union Carbide patent, U.S. Pat. No. 4,426,485 also describes a solvent/water mixture for a polymer thickening composition of appropriate amounts of polymer, the organic solvent diethylene glycol monobutyl ether, and water. This organic solvent as well as other glycols present evaporate after paints and coatings containing such chemicals are applied, and enter the atmosphere during the drying of the aqueous composition. Similar evaporation occurs following the application of joint compounds or cosmetics, among other thickened substances.
The removal of organic vapors released in the course of various industrial and architectural painting and, for example, in the manufacture of painted and coated articles such as automobiles and furniture, has become increasingly important in combating atmospheric pollution. Such organic vapors not only have an offensive odor, but they may also cause damage to vegetation, wildlife and other aspects of the external environment, besides constituting a health hazard. The authorities in many countries, including the United States and Western Europe, have increasingly imposed lower and lower tolerances as to such gases being vented to the atmosphere, and it could become imperative in many countries to remove virtually all such volatile organics, under the penalty of an absolute ban on continuing operation of the impacted business activity. By law passed in 1990, California has forbidden the sale or manufacture of any architectural coating which contains more than a set amount of volatile organic compounds per liter.
The prior art in general does not appear to have recognized the importance of reducing or eliminating volatile components in thickening compositions. A thickening composition having little or no VOC will contribute little or zero VOC to the aqueous system being thickened.
The patent specification of U.S. Pat. No. 5,137,571 recently issued to Rohm & Haas Corp., does exhibit some understanding of the environmental problems which associative thickening compositions can cause. In an effort to eliminate the need for organic cosolvents, the patent describes the use of cyclodextrin compounds and their hydroxyethyl--or hydroxypropyl--derivatives to suppress the viscosity of an associative thickener in water. It has been proposed that the cyclodextrin compounds are absorbed onto the hydrophobic moieties of associative thickeners. A 1993 abstract of work done at North Dakota State University by Glass shows further work on cyclodextrin complexed with surfactants.
Japanese Patent Disclosure 60-49022 discloses that a large list of various chemical additives may, depending on the intended use, be added to associative thickeners, which list includes surfactants, but with no teaching that the use of such materials with water can reduce volatiles, replace organic solvents, or affect the viscosity of the associative thickener in water. U.S. Pat. No. 4,722,962, issued to DeSoto Inc., refers to nonionic surfactants in relation to associative thickeners, but teaches reacting such surfactants into the thickener polymer to form a portion of the final polyurethane. No understanding of any problem relating to organic pollution is demonstrated.
A 1989 Akzo Chemical brochure for commercial products designated Dapral T210 and Dapral T212 describes these products as thickeners for latex formulations, surfactants formulations, water/glycol mixtures and oil-in-water emulsions. Since the two Akzo thickeners are not soluble in water in all proportions, the brochure describes the use of surfactant/thickener premixes as a way of effecting the behavior in aqueous systems of the thickeners. There is again no discussion or understanding of the importance of reducing organic volatile emissions.